Wafer reclamation is becoming more important to integrated circuit manufacturers as the cost of test wafers continues to rise. Wafer reclamation is a process in which layers of films, or "film stacks", on semiconductor wafer substrates are removed so that the wafer can be reused. Of course, in order for wafer reclamation to be practically and economically feasible, the method of removing the film stacks must not only be capable of removing substantially all of the film layers, but must be capable of doing so at a cost less than that of replacing the wafer. Furthermore, the process must not substantially damage the surface of the wafer. Finally, the process of removing the film stacks desirably proceeds at a reasonable rate so that the utilization of the reclamation process is practical in a typical manufacturing setting.
While many kinds of films, such as oxide films, nitride films, and metal films, are relatively easy to selectively remove using conventional reclamation chemicals and techniques, the selective removal of other films, such as tantalum-containing films, can be difficult. In particular, different tantalum-containing films can require different reclamation chemicals to remove them. That is, reclamation chemicals useful to remove one type of tantalum-containing film are not necessarily useful to remove other types of tantalum-containing, films. For example, only one reclamation chemical, concentrated hydrofluoric acid, previously has been identified that is effective to remove tantalum films. However, concentrated hydrofluoric acid will not remove other tantalum-containing films, e.g., tantalum nitride films, and in fact, a mixture of ammonium hydroxide and hydrogen peroxide (ammonium peroxide mixture or APM) is the only currently identified effective reclamation chemical for tantalum nitride films. This presents a processing difficulty in that wafers comprising these different tantalum-containing films must be separated from each other before being subjected to the appropriate reclamation process. Such separation can be time consuming to carry out, and in the least represents the addition of a step that may undesirably lengthen the reclamation process.
Additionally, not only can various tantalum-containing films require the use of different reclamation chemicals to remove them effectively, but the different film layers typically present in film stacks comprising tantalum-containing films can also require the use of several different reclamation chemicals to effectively reclaim a wafer comprising such a film stack. That is, tantalum-containing films are typically used in film stacks comprising copper (Cu) overlying the tantalum-containing film, which, in turn, overlies a silicon oxide (SiO.sub.2) film, such as thermal oxide (TOX) or tetraethyl orthosilicate (TEOS), e.g., Cu/Ta or TaN/TEOS or TOX film stacks. Each of these individual films typically requires the use of a different reclamation chemical in order to remove the film effectively. This not only has the effect of rendering the reclamation process of wafers comprising such film stacks time consuming in that the process must incorporate the time necessary to change chemicals, but also can render the reclamation process more expensive than a corresponding reclamation process requiring fewer chemicals.
For example, one process currently employed for the removal of a copper/tantalum/TEOS film stack comprises the application of a hydrochloric acid and hydrogen peroxide mixture (HPM), followed by the application of concentrated hydrofluoric acid (HF). In this process, the HPM acts to remove substantially all of the copper, while the concentrated HF is effective to remove both the tantalum and the TEOS. Although extremely effective for these particular film stacks, this process has some concurrent effects that can be undesirable in some applications. For example, concentrated HF etches oxide rapidly, and thus, may increase the surface roughness of the wafer if applied for too lengthy a period of time. Additionally, concentrated HF may not be compatible with materials desirably used to fabricate some kinds of reclamation process equipment. For example, contact with concentrated HF may result in the undesirable discoloration of components of reclamation equipment fabricated from polyvinylidene fluoride (PVDF), perfluoroalkoxy (PFA) or polytetrafluoroethylene (PTFE).
An additional conventional reclamation process, effective to remove copper/tantalum nitride/TEOS or TOX film stacks, involves the application of HPM followed by the application of APM followed by the application of dilute HF. In this process, the HPM again functions to remove substantially all of the copper layer, while the APM acts to remove the tantalum nitride (TaN) and the dilute hydrofluoric acid removes the TEOS layer. This process is less than ideal in that the use of three chemicals is required in order to effectively remove the entire film stack. Additionally, in order to achieve an etch rate of tantalum nitride that is practical for use in a reclamation process, the APM typically must be heated. Heating the APM necessitates the inclusion of a heater in the reclamation processing system, which may present additional cost and space issues. Additionally, relatively hot processing in a manufacturing setting is generally considered to be less desirable than ambient processes in that the handling of heated chemicals is more challenging than is handling the same or different chemicals at ambient temperature.
An additional complication in the reclamation of wafers bearing film stacks comprising tantalum-containing films is presented by the inclusion of TIOS as the oxide layer in such film stacks. That is, although many oxide film layers are typically applied to wafers by subjecting the wafers to high heat, which results in the oxide film layer forming on both the top and bottom surface of the wafers, TEOS films are not formed or applied in this manner. Rather, TEOS films are applied by chemical vapor deposition, which results in the film being formed only on the top surface of the wafer. Thus, whereas wafers bearing film stacks on their top surface comprising other oxide films typically have at least some amount of oxide on their bottom surface that thereby acts to protect this bottom surface from the reclamation chemicals that may otherwise damage the bare silicon wafer, wafers bearing film stacks comprising TEOS as an oxide layer do not have such a protective oxide film on their bottom surface. As a result, the exposed bottom surface of such wafers can be damaged by the application of certain reclamation chemicals, and in particular, can be damaged by the application of reclamation chemicals comprising a fluoride ion source.
Aqueous compositions including nitric acid (HNO.sub.3), hydrofluoric acid (HF), and water (H.sub.2 O) have been used in the past in the manufacture of integrated circuits, particularly for silicon etching processes. Conventionally. these compositions have included from about 50% to about 60% by volume of aqueous HNO.sub.3 (typically about 70 weight % solids), 10% or more aqueous HF (typically about 50 weight % solids), with the remainder being H.sub.2 O. See, for example, "Silicon Processing for the VLSI era--Volume 1, process technology", by S. Wolf and R. N. Tauber, Lattice Press. Sunset Beach, Calif., page 531-532, 1986. However, and although these compositions have not been studied for use in etching applications where it is desired to remove tantalum-containing films, because of the extremely fast etch rates these compositions exhibit with oxide films, it is expected that these compositions will etch oxide films much faster than tantalum-containing films. In other words, these compositions are expected to exhibit selectivity that favors oxide, not tantalum-containing films. In reclamation processes, where an oxide layer desirably acts as a "stop layer", this lack of stripping selectivity between tantalum-containing films and oxide films would be undesirable. These compositions would most likely etch through the oxide before all the tantalum is removed, thereby damaging the underlying wafer. Additionally, the use of such compositions to reclaim wafers bearing a film stack comprising TEOS can result in damage to the bottom, unprotected surface of the wafer due to the presence of a relatively high amount of concentrated hydrofluoric acid. As a result, these aqueous HF/HNO.sub.3 compositions are not considered to be useful for reclamation applications, and in particular, in reclamation processes desirably utilized to reclaim wafers bearing tantalum-containing film stacks.
What is needed, therefore, are reclamation compositions and methods that provide the desired stripping selectivity between tantalum-containing films and oxide films, i.e., in which the etching selectivity favors tantalum-containing films so that the oxide layer can act as a stop layer to protect the wafer as the tantalum-containing film is removed. Such compositions and processes desirably would be useful for film stacks comprising any type of tantalum-containing films, and further would be suitable for use with film stacks comprising any oxide film, including TEOS, without causing substantial damage to the bottom surface of the wafer. It would further be desirable for such compositions to be compatible with materials such as PVDF that may be used in the manufacture of reclamation process equipment. Finally, it would be advantageous if such reclamation compositions and processes would be effective at substantially ambient temperatures so that the cost and safety concerns that are sometimes associated with hot chemical processes could be avoided.